The present invention relates to oriented nylon casings, particularly food casings suitable for making sausage.
Tubular films are used as sausage casings for processing and packaging cooked sausages including water cooked or steam cooked sausages such as liver sausage and fleischwurst or cheese sausage (cheese packed in the shape of a sausage).
It is generally known that selection of films for packaging food products such as meat and cheese sausages includes consideration of one or more criteria such as cost, abrasion resistance, wrinkle resistance, meat adhesion, dimensional uniformity and stability, stiffness, strength, printability, durability, oxygen and water barrier properties, stretchability, machinability, optical properties such as haze, gloss and freedom from streaks and gels, and safety for contact with food.
In general, commercial sausage making operations for making water cooked or steam cooked sausages require casings made from materials able to perform well in the following typical process steps:
1. Stuffing with meat emulsion to a uniform diameter;
2. Clipping or otherwise sealing the casing about its circumference to form discrete logs;
3. Cooking the encased sausage to temperatures of at least 65.degree.-100.degree. C.;
4. Chilling the cooked encased sausages e.g. to temperatures as low as 4.degree. C. or less;
5. Optionally cutting the logs into discrete lengths or slices; and
6. Repackaging cut logs or slices e.g. by vacuum packaging.
Various monolayer and multilayer casings have been proposed and used commercially to make water cooked or steamed cooked sausages such as fleischwurst and liver sausage. These sausages are typically made in casings having moisture barrier properties to prevent loss of water during and after cooking. The palatability of these sausages may be altered by changes in their moisture content. Excess water absorption may dilute the flavor and texture of the sausage, while loss of moisture may dry out the sausage and adversely affect both the texture and flavor. Also, sausages are traditionally sold by weight and any loss of moisture results in a loss of weight and may reduce profit to the seller.
Desirably, casings for these types of sausages will also have low oxygen permeability to avoid discoloration, adverse flavor changes and oxidation of the sausage during storage. Liver sausage in particular is easily susceptible to defects when contacted with excessive oxygen and discoloration causing an unappetizing appearance may be a particularly acute problem for this product.
Furthermore, it is highly desirable to produce an encased cooked sausage which exhibits a tight fitting casing having few or no wrinkles even after prolonged storage. There should be a minimum of spaces or pockets between the sausage mass and the inside of the casing since such spaces or pockets promote separation and collection of fats, liquid and gelatinous materials in such spaces which leads to a non-uniform sausage appearance which is unappetizing and undesirable to consumers.
Cellulose casings of e.g. fiber reinforced regenerated cellulose coated with moisture barrier coatings such as a polyvinylidene chloride copolymer (PVDC) e.g. saran, have been commercialized as have monolayer casings made of polyvinylidene chloride copolymers such as saran. These casings have excellent oxygen and moisture barrier properties.
Also, commercially available coated cellulosic casings have excellent dimensional uniformity and stability, but disadvantageously are expensive to produce compared to plastic casings. Also, use of polyvinylidene chloride copolymers such as saran has raised environmental concerns due to the difficulties of recycling chlorinated polymers and possible release of chlorinated by-products during incineration. Furthermore, the dimensional stability and uniformity of saran monolayer casings are generally inferior to the cellulosic casings, and saran monolayer casings after cooking and chilling tend to relax causing a wrinkled appearance.
Plastic casings are typically made by either blown film or oriented film processes. Biaxially stretched sausage casings of polyethylene terephthalate (PETP) have been made and are known in the art as well as biaxially stretched casings of polyvinylidene chloride copolymers (PVDC). These casings reportedly have much improved tensile strength and increased dimensional stability during stuffing compared to unstretched plastic sausage casings made from blown film. However, these stretched casings result in wrinkle-free sausages only if the encased sausages are subjected, after cooking and cooling, to an additional heat treatment which is known in the art as "after-shrinking". "After-shrinking" means that the cooked and cooled sausage (approximately 4.degree. C.) is heated to at least about 80.degree. C. or higher for a few seconds in a hot water bath or by hot air treatment. During this heat treatment, the casing shrinks and lies more closely and wrinkle-free against the encased sausage meat, the volume of which had been previously reduced by cooling. This additional after-shrinking step is not required in the manufacturing process for boiling and scalding sausages using cellulosic casings. Consequently, this subsequent processing step, which requires additional equipment, energy, time and money is undesirable. In addition to wrinkle-formation, biaxially stretched sausage casings of PETP and PVDC also reportedly show undesirable deposits of fat, liquid or gelatinous material which collects in pockets or spaces between the sausage material and the sausage casing thereby producing an undesirable appearance to the consumer.
To ameliorate the aforementioned problems and costs associated with coated cellulosic casings and PVDC-type casings, several polyamide casings have been introduced into the market. Both monolayer and multilayer polyamide casings have been commercialized and both non-shrinkable and shrinkable casings have been utilized made by blown film and oriented film processes.
As discussed in U.S. Pat. No. 4,303,711 "single-layer, unstretched plastic casings consisting of higher homopolyamides (polyamide 11 and polyamide 12)" are known as well as plastic casings consisting of such polyamides coextruded in two layers with polyamide 6 as the outer layer. These polyamide casings may be easily made by the blown film technique, but typically suffer from a lack of dimensional stability and uniformity, being deformed upon stuffing so that production of stuffed sausages to a uniform diameter is difficult. The '711 patent further indicates that these unstretched films suffer from an undesirably wrinkled appearance after cooking and chilling.
As noted above, to overcome the defects or inadequate performance of seamless thermoplastic casings produced by blown film technology, plastic casings have been produced utilizing stretch orientation.
Also, various attempts have been made at making stretch oriented polyamide casings. Uniaxially stretched casings which are only stretched in the longitudinal (machine) direction reportedly have the same disadvantages as unstretched casings regarding insufficient dimensional stability, nonuniformity of diameter and excessive wrinkling.
Erk et al. U.S. Pat. No. 4,560,520 discloses forming multiaxially stretched, monolayer, polyamide, tubular films e.g. of nylon 6 or nylon 66 which have elastic properties and which are said to be used for packaging table sausages and boiled sausages. The disclosed films are "fixed thermally" and shrunk after stretching e.g. by subjecting the tube to controlled shrinkage of at least 15% and at most 40% at temperatures above 90.degree. C. and also subjecting the film to infrared irradiation. This is to produce a nylon casing which does not have shrinkage at temperatures under 90.degree. C. This pre-shrunk casing is used for stuffing with meat emulsion and relies upon its elastic properties to provide wrinkle resistance.
The above '520 patent refers to the wrinkle problem associated with use of thermoplastic casings and states that:
"nearly all of the thermoplastic materials used as sausage casings have the disadvantage that once the sausage has been boiled and subsequently cooled they do not fit round the sausage material closely, in the manner of a natural casing, but appear to be more or less wrinkled. The customer equates such wrinkled appearance with old goods that are no longer fresh, and this is an obstacle to sale. For this reason up to now such sausage casings of thermoplastic materials have only been used by sausage makers for second-rate goods".
This patent goes on to refer to a "shrinkable multiaxially stretched thermally fixed sausage casing of polyamide" which remains close fitting but suffers from insufficient resistance to tearing. The assignee of this patent, Naturin-Werk Becker & Company has commercialized several monolayer nylon casings under the trademarks Optan, Betan and Tripan.
Nylon monolayer casings whether made by the blown film process or the stretch oriented film process are disadvantageously sensitive to moisture. As noted above it is desirable for casings used to package fleischwurst and liver sausage type products to have low permeability to steam or water. Also, moisture is known to adversely affect the oxygen barrier properties of many nylons causing an undesirable increase in oxygen transmission rates when wet. In view of these disadvantages, attempts have been made to utilize blends of nylon with other materials to enhance properties such as gas and water vapor impermeability. For example, U.S. Pat. No. 4,303,711 discloses a plastic casing made from a mixture of polyamide and ionomer. Also, the company Hoechst AG has commercialized what are believed to be monolayer casings containing blends of polyamide and polyester.
Furthermore, attempts have been made to use nylon in biaxially stretched multilayer casing to overcome these disadvantages. For example, U.S. Pat. No. 4,888,223 discloses 2 to 5 layer heat shrinkable tubular structures all having polyamide in either the outer layer or core layer and having a polyolefin inner layer that is corona treated for meat adhesion.
Also, U.S. Pat. No. 4,855,183 discloses a multilayer tubular shrink film having a polyamide inner layer which is irradiated to promote meat adhesion and which has additional polyolefinic layers which may comprise materials such as EVA, EMA, EEA, LLDPE, VLDPE, LDPE, HDPE or MDPE.
Disadvantageously, the above '223 and '183 patents disclose structures that require a corona treatment or irradiation step to enhance meat adhesion thereby requiring additional equipment, processing time and/or cost.
Without admitting that it is prior art, it is noted that European Patent Document No. 467,039 discloses a multilayer casing which may consist of a three layer coextruded and biaxially oriented tube which, if desired, may be "thermofixated". A structure having inner and outer polyamide layers separated by a middle layer of polyolefin which blended with or coated with an adhesion imparting component is disclosed for use as a sausage casing with low permeability to steam and oxygen. The middle layer is preferably a polyolefin blended with an adhesion imparting component in a portion which in general is 5-50 weight percent based on the polymer blend present in the middle layer. The polyolefin is stated to be usually a homopolymer of ethylene or propylene or a copolymer of linear alpha-olefins having 2 to 8 carbon atoms or a blend thereof. C.sub.2 /C.sub.3 and C.sub.3 /C.sub.4 copolymers are said to be useful as well as C.sub.2 /C.sub.3 /C.sub.4 terpolymers or a blend of a C.sub.3 /C.sub.4 copolymer with C.sub.2 /C.sub.3 /C.sub.4 terpolymer. The two examples disclosed both specify HDPE. According to this document the portion of the adhesion imparting component is preferably 10 to 35% by weight but "should be kept as low as possible". This adhesion component is viewed as being required to prevent layer separation during cooking in hot water. Useful adhesion imparting agents are disclosed as including polyolefin resin modified with functional groups such as vinyl acetate, acrylic acid and methacrylic acid as well as their esters and salts and furthermore ethylenically unsaturated carboxylic anhydride groups. These casings are oriented by biaxial stretching. To improve dimensional stability after stretching, the casing is annealed to produce a material having a shrinkage of less than 20%, in particular less than 15% in both vertical and horizontal directions at temperatures of up to 90.degree. C.
The above mentioned EP Document No. 467,039 discloses preparation of its casing by reference to EP 305 874 (which corresponds to U.S. Pat. No. 4,886,634), which discloses a rather complicated apparatus employing e.g. a vacuum tank and probe pipe with a sealing element. Also, blending polyester with the polyamide is suggested to facilitate biaxial stretching.
Disadvantageously, orientation of seamless tubes of nylon by biaxial stretching is difficult. Extrusion and orientation of multilayer tubes, especially coextruded tubes, containing mixed layers of polyamides and other materials having different melting points, melt viscosities, and a different affinity for water can be very difficult. For example, Hisazumi et al. U.S. Pat. No. 4,892,765 notes that although it is desirable to extrude films for packaging hams and sausages in tubular form, it is difficult to make a stretched tubular polyamide film of uniform thickness. This patent also notes that layer adhesion becomes weak when multilayer, polyamide films are stretched. Hisazumi et al. disclose production of a heat shrinkable multilayer film having a core layer of a polyvinylidene chloride copolymer attached to opposing polyamide layers (e.g. of nylon 6/66 copolymer) by opposing adhesive layers. This film is made using an orientation process which utilizes water to soften and plasticize the nylon to a degree sufficient to allow or facilitate orientation. In order to condition nylon with water prior to orientation the primary extruded tube is exteriorly and interiorly treated with water. In forming multilayer films having a nylon interior layer such conditioning of the interior layer is difficult particularly where relatively small diameters of primary tubing is used (e.g. less than 1.9 inches (4.8 cm)). Introduction of water to the inside of the tube may generally be accomplished either through the die, by perforating the casing and introducing a trapped slug of water or through permeation across the casing wall from the outside of the tube. Disadvantageously, introduction of water through the die is difficult because temperatures of the die and the extruding polymer melt are generally above the boiling point of water causing vaporization. Also, introducing a slug of water through a perforation in the tube wall creates waste in the area of perforation and makes that portion of the primary tube difficult if not impossible to stretch orient. Also, small diameter casing may only hold a small volume of water which is quickly absorbed thereby necessitating addition of further slugs which disrupt and slow operations in addition to creating additional waste. Also, multilayer films containing a moisture barrier layer or having a polyamide blended with a moisture barrier resin made it difficult if not impossible to condition by moisture permeation across the moisture barrier. Generally, attempts at biaxially stretching nylon multilayer casing have utilized polyamide as the outer layer of the tube. The orientation processes employed for nylon multilayer casing have tended to involve complicated apparatus and processing such as that found in U.S. Pat. No. 4,886,634.
Also, coextrusion of polyvinylidene chloride copolymers (PVDC) with polyamides or polyolefins is difficult because polyvinylidene chloride copolymers are very termperature and shear sensitive during extrusion. These copolymers are extrudable only over a narrow temperature range without causing degradation of the polymer in the extruder or die. Thermal degradation of polyvinylidene chloride copolymers form particles or gels of degraded material which may exit the die and cause imperfections in the film. Even at optimum extrusion temperatures, a certain amount of degraded PVDC material will form in the extruder and die, necessitating periodic cleaning and removal from service of the equipment. Polyamides require much higher temperatures for extrusion generally about 200.degree. C. or higher. At these higher extruder and die temperatures coextrusion of polyamide with PVDC is difficult and degradation of the PVDC may be expected even if the PVDC layer is insulated from polyamide layers by intermediate layers. Resulting imperfections in the film may detrimentally affect the appearance, strength and barrier properties of the film and/or ease of orientation or biaxial stretching.
In summary, although several of the aforementioned plastic casing products have gained varying degrees of commercial acceptance in different market segments, their advantage compared to the traditional cellulosic casing has been chiefly one of cost with the problems of dimensional stability, uniformity of diameter, and wrinkling being persistent concerns.
Prior art fiber reinforced cellulose casings coated with moisture barrier coatings perform well in processing water/steam cooked sausages such as fleischwurst and liver sausage. However, the high cost of manufacture of such casings has led casing manufacturers to search for less expensive alternatives. Thermoplastic films of various compositions have been suggested and some have found varying degrees of success in various segments of the market. Thermoplastic sheet film has been made into a tube by seaming, but this is a difficult process which produces a casing having a seamed area which may undesirably differ in appearance and performance relative to an unseamed casing.
Seamless tubular thermoplastic casings have been made which overcome the objections to seamed casings. Various materials have been employed, but materials containing chlorinated polymers have been objected to for environmental reasons among others. Seamless polyamide casings have been made of blown film, however these casings tend to have poor performance with respect to wrinkling, uniformity of diameter, and dimensional stability. Seamless biaxially oriented multilayer films have also been made, however, such films have been difficult to produce requiring special blend formulations and structures or complicated equipment and procedures.
Therefore, it is an object of the present invention to provide a multilayer, biaxially stretched, heat shrinkable, thermoplastic film useful as a casing for making sausages such as fleischwurst or liver sausage which includes among its desirable properties one or more, preferably a combination, of the following:
a) sufficient flexibility and softness to facilitate shirring and subsequent stuffed log formation by gathering of the casing and clipping to form the sausage ends;
b) resistance to permanent deformation during stuffing, cooking and chilling, and maintenance of a symmetrical cylindrical shape with a minimum of curvature or bulging;
c) an ability to adhere to the meat during expansion and contraction of the sausage during cooking and chilling;
d) resistance to bursting or tearing during stuffing, during cooking at elevated temperatures, and during subsequent handling;
e) resistance to wrinkling during processing and handling;
f) little or no moisture loss during cooking and storage i.e., high cooking yield;
g) resistance to passage of oxygen in order to prevent spoilage; and
h) an ability to be cut or sliced easily without edge curling or splitting.
It is a further object of the present invention to provide a tubular film having a unique combination of shrink, mechanical strength and barrier properties suitable for use as a sausage casing.
It is a further object of the present invention to provide a polyamide sausage casing having a polyamide inner layer which adheres to meat without requiring addition of starch based additives or treatment with electron beam irradiation or corona discharge.
It is a further object of the present invention to provide a multilayer oriented structure having sufficient shrinkage values and shrink force values to provide good conformation of the casing to the filling after cooking, chilling and storage.
It is a further object of the present invention to provide an improved biaxially stretched, heat shrinkable polyamide containing multilayer casing.
It is a further object of the invention to provide a casing having the above properties at a minimum of cost.
A further object of the invention is to provide a process of manufacture which is as simple and inexpensive as possible consistent with production of a film casing having the desired performance characteristics.